Thermal spreader using thermal conduits

ABSTRACT

A thermal spreading device disposable between electronic circuitry and a heat sink includes a substrate having parallel first and second faces and conduits extending through the substrate between the faces. The substrate material has a first thermal conductivity value in a direction parallel to the faces and a second thermal conductivity value in a direction normal to the faces, with the second thermal conductivity value being less than the first thermal conductivity value. The conduit material has a thermal conductivity value associated with it, with the thermal conductivity value being greater than the second thermal conductivity value of the substrate. One method of fabricating the thermal spreading device includes disposing a molding material radially about the rods and hardening the material. Other methods include press fitting and shrink fitting the rods into a substrate material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/953,991 filed Sep. 29, 2004, which is a divisional of U.S. Pat. No.6,821,624 issued on Nov. 23, 2004, the contents of which areincorporated by reference herein in their entirety.

BACKGROUND

In nearly every sector of the electronics industry, electronic circuitryinvolves the interconnection of an integrated chip (hereinafter “chip”)and a surface or device upon which the chip is supported. Duringoperation of the circuitry, heat is generated and a heat flux isestablished between the chip and its environment. In order to removeheat more effectively to ensure the proper functioning of the circuitry,the heat flux is disseminated across a surface area larger than thesurface area of the chip and transferred to an attached heat sinkdevice. Once the heat is transferred to the heat sink device, it can beremoved by a forced convection of air or other cooling means.

In some applications, multiple processors and their associated controland support circuitry are arranged on a single chip. Such arrangementsmay result not only in a further increase in the heat flux, but also ina non-uniform distribution of the heat flux across the surface of thechip. The non-uniformity of the distribution of the heat flux isgenerally such that a higher heat flux is realized in the processor coreregion and a significantly lower heat flux is realized in the region ofthe chip at which the control and support circuitry is disposed. Thehigh heat flux in the processor core region may cause devices in thisregion to exceed their allowable operating temperatures. The resultingdisparity in temperature between the two regions, which may besignificant, may contribute to the stressing and fatigue of the chip.

A thermally conductive heat spreading device is oftentimes disposedbetween the chip and the heat sink device to facilitate thedissemination of heat from the chip. Such heat spreading devices aregenerally plate-like in structure and homogenous in composition andfabricated from materials such as copper, aluminum nitride, or siliconcarbide. Newer carbon fiber composites exhibit even higher thermalconductivities than these traditional thermal spreader materials;however, they tend to be anisotropic in nature, exhibiting widevariations in thermal conductivity between a major axis normal to theface of the structure (in the Z direction) and the axes orthogonal tothe major axis (in the X and Y directions). Moreover, the lower thermalconductivity in the direction along the major axis tends to have theeffect of increasing the thermal resistance of the heat spreadingdevice, thereby inhibiting the dissemination of heat from the device.

SUMMARY

A thermal spreading device disposable between electronic circuitry and aheat sink is disclosed. The device includes a substrate having a firstface and a second face and a plurality of conduits extending through thesubstrate from the first face to the second face. The two faces of thesubstrate are disposed in a parallel relationship. The material of whichthe substrate is fabricated has a first thermal conductivity value in adirection parallel to the faces and a second thermal conductivity valuein a direction normal to the faces, with the second thermal conductivityvalue being less than the first thermal conductivity value. The materialof which each conduit is fabricated has a thermal conductivity valueassociated with it, with the thermal conductivity value of each conduitbeing greater than the second thermal conductivity value of thesubstrate.

One method of fabricating the thermal spreading device includesarranging a plurality of thermally conductive rods such that the rodsextend longitudinally in a common direction, disposing a moldingmaterial radially about the longitudinally extending rods, hardening themolding material around the plurality of thermally conductive rods, andcutting the hardened molding material into slices in a directionperpendicular to the direction in which the rods longitudinally extend.Other methods of fabrication include press fitting or shrink fitting thethermally conductive rods into holes in the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood by those skilled in thepertinent art by referencing the accompanying drawings, where likeelements are numbered alike in the several FIGURES, in which:

FIG. 1 is a perspective cutaway view of a thermal spreading device;

FIGS. 2A through 2C are perspective views of a batch process of thefabrication of a thermal spreading device;

FIGS. 3A and 3B are perspective views of a batch process of thefabrication of a thermal spreading device in which conduits are pressfitted into the substrate;

FIG. 4 is a sectional view of a step in a batch process of thefabrication of a thermal spreading device in which conduits are shrinkfitted into the substrate;

FIG. 5 is a sectional view of the engagement of a thermal spreadingdevice with a chip and a heat sink;

FIGS. 6 and 7 are plan and cross sectional views of an alternateexemplary embodiment of a thermal spreading device; and

FIG. 8 is an exploded perspective view of the engagement of the thermalspreading device of FIGS. 6 and 7 with a chip.

DETAILED DESCRIPTION

Referring now to FIG. 1, an exemplary embodiment of a thermal spreadingdevice is shown generally at 10 and is hereinafter referred to as“thermal spreader 10.” Thermal spreader 10 is a conduction medium thatprovides for thermal communication between electronic circuitry (e.g., achip) and an environment to which thermal spreader 10 is exposed. Thethermal communication is effectuated by the conduction of heat across asubstrate 12 to a heat sink (shown with reference to FIG. 5). Becausethe materials from which substrate 12 are fabricated are generally of ananisotropic nature, substrate 12 is oftentimes characterized by a markeddisparity in thermal conductivities in orthogonal directions. Inparticular, the thermal conductivity of substrate 12 in a directionshown by an arrow 16 (Z direction), which is normal to the interface ofthermal spreader 10 and the circuitry (not shown), may be substantiallyless than thermal conductivities in the directions shown by an arrow 18(X direction) and an arrow 19 (Y direction) along the same interface ofthermal spreader 10 and the circuitry. Due to such disparities, thethermal resistance across substrate 12 (in the direction of arrow 16) isincreased, and the rate of heat transfer (flux) across thermal spreader10 varies dramatically from the flux in the direction (as shown byarrows 18 and 19) that the interface extends.

In order to enhance the thermal communication across thermal spreader10, substrate 12 is configured to include thermal conduits 14. Thematerials from which thermal conduits 14 are fabricated generally havethermal conductivity values that are substantially higher than thethermal conductivity values in the Z direction of the material fromwhich substrate 12 is fabricated. Because the flux through conduits 14is greater than the flux in the same direction across the surroundingsubstrate 12, heat conduction is enhanced across substrate 12 in thedirection shown by arrow 16 (Z direction), viz., in the direction inwhich conduits 14 extend. Heat transfer is thereby optimized throughsubstrate 12 via conduits 14.

Conduits 14 are defined by rods or wires having substantially circularcross sectional geometries, as is shown. Rods or wires havingsubstantially circular cross sectional geometries enable a substantiallyuniform transfer of heat to be maintained in the directions radial tothe circular cross section. Other cross sectional geometries that may beused include, but are not limited to, elliptical, square, flat,multi-faced, and configurations incorporating combinations of theforegoing geometries. Regardless of the cross sectional geometry,conduits 14 are formed from materials having high thermalconductivities. Such materials include, but are not limited to, copper,aluminum, carbon, carbon composites, and similar materials that exhibita high thermal conductivity along the conduit axis. The carbon materialsmay be fibrous or particulate in structure.

Substrate 12 provides an anchor into which conduits 14 are disposedwhile further providing a medium for the transfer of heat in directionsalong and parallel to the interface defined by the positioning ofthermal spreader 10 on the chip. Exemplary materials from whichsubstrate 12 can be fabricated include, but are not limited to, carbonand carbon composites. As noted above with respect to conduits 14, thecarbon materials may be fibrous or particulate in structure.

The configuration of thermal spreader 10 is generally such that conduits14 are arranged to be parallel to each other, as is shown in FIG. 1.Furthermore, conduits 14 generally extend linearly between opposingsurfaces of substrate 12. As shown, the architecture of thermal spreader10 is further defined by a substantially uniform spatial positioning ofconduits 14 over any randomly selected section of substrate 12. The evendistribution of conduits 14 facilitates and improves the conduction ofheat from a first face 20 disposed adjacent the chip and anopposingly-positioned second face 24 disposed adjacent the heat sink.Such a distribution provides for the effective transfer of heatlongitudinally through conduits 14 while maintaining the substantiallyuniform transfer of the heat in the directions radial to the surfaces ofconduits 14.

When thermal spreader 10 is mounted between a chip (shown with referenceto FIG. 5) and a heat sink (also shown with reference to FIG. 5),conduits 14 enable heat generated during the operation of the chip to becommunicated from first face 20 of thermal spreader 10 through conduits14 across substrate 12 to second face 24 of thermal spreader 10.Although the material of which substrate 12 is fabricated allows forsome degree of thermal conduction between faces 20, 24, the anisotropicnature of the material causes heat generated by the chip and transferredto thermal spreader 10 to be more substantially dissipated throughsubstrate 12 in the directions shown by arrows 18 and 19. Dissipation ofheat in the directions shown by arrows 18 and 19 allows for the heat tobe conducted to a larger number of conduits 14, which further allows forthe more effective transfer of heat from the chip to the heat sink.

Referring now to FIGS. 2A through 2C, an exemplary batch processillustrating the fabrication of the thermal spreader is illustrated. Theprocess comprises arranging the rods or wires by which conduits 14 aredefined into an array, which is shown generally at 30 in FIG. 2A. Therods are arranged such that the longitudinal axes of the rods areparallel to each other and held fast by a jig (not shown) or otherdevice configured to maintain the rods in their proper alignment.Molding material of which the substrate is formed is then disposedaround the rods, hardened, and cured, as is shown in FIG. 2B. Thehardened and cured molding material forms a block, shown generally at32, having thermal conduits 14 extending between first face 20 andopposing second face 24 thereof Block 32 is then sawed or otherwise madeinto sheets 34, as is illustrated in FIG. 2C. Each sheet 34 is of athickness t_(s), which is slightly in excess of the desired thickness ofthe finished thermal dissipating device. Sheets 34 are then polished onat least one face thereof to bring thicknesses t_(s) within theallowable tolerances of final product. Polishing of the sheets on bothsides further provides sheets 34 with surface textures conducive to amore effective transfer of heat between the chip and the heat sink.Finally, sheets 34 are cut into individual thermal spreaders 10 of thedesired length and width.

In another exemplary process of the fabrication of the thermal spreader,thermal conduits 14 may be press-fitted into substrate 12, as is shownin FIGS. 3A and 3B. Referring to FIG. 3A, holes 28 are drilled, punched,or otherwise formed in block 32. The cross sectional geometries of holes28 correspond with the cross sectional geometries of conduits 14insertable into holes 28. Referring now to FIG. 3B, conduits 14 areinserted into holes 28 under a compressive force Cf effectuated by apress (not shown) or a similar apparatus. The mechanical tolerances ofconduits 14 are such that when conduits 14 are received in holes 28, atight fit is maintained between the inner surfaces of holes 28 and theouter surfaces of each conduit 14, thereby allowing effective thermalcommunication to be maintained between the material of block 32 andconduits 14. Block 32 may then be sawed or otherwise formed into sheetsand polished and cut to the desired lengths and widths.

In yet another exemplary process of the fabrication of the thermalspreader, thermal conduits 14 may be shrink-fitted into substrate 12, asis shown in FIG. 4. In the shrink-fitting process, holes 28 are againdrilled, punched, or otherwise formed in block 32, as was describedabove. Block 32 is heated to a temperature that causes block 32 (andsubsequently holes 28) to expand. Upon expansion, conduits 14 areinserted into holes 28 with little effort such that space is defined byinner surfaces 34 of holes and outer surfaces 36 of conduits 14. Block32 is then cooled to cause the material of fabrication of block 32 tocontract, thereby constricting holes 28 and eliminating the spacesdefined between the inner surfaces of holes 28 and the outer surfaces ofconduits 14. Once constricted, conduits 14 are securely retained withinblock 28. Block 32 may then be sawed or otherwise formed into sheets andpolished and cut to the desired dimensions in manners similar to thosedescribed above to form the final product.

Referring now to FIG. 5, a thermal conduction package is shown generallyat 38. In thermal conduction package 38, thermal spreader 10 is shown asit would be disposed between the chip 40 disposed in electroniccommunication with its associated circuitry through substrate 42 and theheat sink 44. Thermal spreader 10 is adhered to chip 40 with an adhesive48, which may be a solder or an epoxy material applied to chip 40 as athin layer upon which thermal spreader 10 is placed. A layer of thermalpaste 50, which is typically a natural or synthetic oil-based compoundwith thermally conductive particle filler, is applied to the exposedsurface of thermal spreader 10 upon which heat sink 44 is mounted. Bothadhesive 48 and thermal paste 50 facilitate the transfer of heat betweenchip 40 and thermal spreader 10 and thermal spreader 10 and heat sink 44respectively, thereby enhancing the conduction of heat across thermalspreader 10.

As is shown with reference to FIGS. 6 and 7, another exemplaryembodiment of a thermal dissipating device is shown generally at 110.Thermal spreader 110 is substantially similar to thermal spreader 10 asillustrated above with reference to FIGS. 1 through 5. Thermal spreader110, however, includes an arrangement of variably spaced conduits 114disposed within a dissipating substrate 112. The arrangement of variablyspaced conduits 114 is configured to define regions 150 in which thedensity of conduits 114 is greater than the density of conduits 114 inadjacently positioned regions 152 of the same substrate 112. Thehigh-density regions 150 are positioned on substrate 112 to registerwith areas of high heat flux on a chip upon assembly of the thermalconduction package.

Referring now to FIG. 8, the engagement of the thermal spreader with thechip is illustrated generally at 138. When thermal spreader 110 isplaced in communication with chip 140, the high-density regions 150register with the areas of high flux 160 on chip 140. Such a placementallows for the increased transfer of heat from the areas of high flux160 on chip 140 to high-density regions 150 of thermal spreader 110while simultaneously providing a thermally adequate transfer of heatfrom the areas of chip 140 from which lower heat flux is realized. Thedisparities in the densities of the conduits in each region 150, 152 areengineered to provide for the removal of heat from each portion of chip140 and the transfer of heat to the heat sink to minimize disparity inheat build up at the interface of chip 140 and thermal spreader 110.Minimization of such disparity may provide improved operability of chip140 and increase the useful life thereof Fabrication of thermal spreader110 is effectuated in a batch process substantially similar to thatillustrated in FIGS. 2A through 4 for thermal spreader 10.

While the disclosure has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1. A method of fabricating a thermal spreading device, the methodcomprising: forming holes into a substrate; heating the substrate toenlarge the holes; inserting a thermally conductive rod into each of theholes; and cooling the substrate to reduce the cross sectional area ofthe holes, thereby causing the rods to be retained in the holes.
 2. Themethod of claim 1 further comprising cutting the substrate into slicesin a direction perpendicular to the direction in which the rods extend.3. The method of claim 2 further comprising polishing a face of the cutslice of substrate.